The behavior of a carbonate sand subjected to a wide strain range of medium-frequency flexural excitation

Abstract

Carbonate sands comprise typical foundation soils for offshore engineering structures such as oil platforms, found in natural deposits. These sands have a high shear strength but also very high compressibility which characteristics are due to the shape and the low tensile strength of the grains. For the seismic design of offshore structures, two key properties comprise the stiffness and damping of the subsoil which depend on the porosity, the level of the confining pressure and the strain amplitude. In this article, the small-strain Young’s modulus (Emax), small-strain damping (Df,min) and the strain-dependent Young’s modulus and damping of a carbonate sand were studied in the laboratory through resonant column tests in flexural mode of vibration. This mode applies a medium-frequency excitation to the sample, typically in the range of 30–80 Hz. The samples were prepared in a dry state and tested under isotropic conditions of the confining pressure with (p′) varying from about 25 to 800 kPa. The small-strain dynamic properties were analysed by means of power law type formulae and the sensitivity of Emax and Df,min to pressure was quantified and discussed. The results indicated that the flexural damping values were much greater in comparison to corresponding results in the literature derived from torsional resonant column tests on sands. This observation is in alignment with a recent study on the flexural response of quartz sands. Overall, the sensitivity of stiffness to pressure was less pronounced than that of reported data in the literature. The medium-strain tests showed a similar trend with respect to the stiffness degradation and damping increase as in the case of torsional resonant column tests reported in the literature for granular soils with a slight dependency of the medium-strain properties to the confining pressure level.